There are numerous portable appliances in use today which may conveniently be powered or have their batteries charged from a motor vehicle electrical system. Such appliances include cellular telephones, two-way radios, video camcorders, and video games.
It is fairly common practice to plug into the cigarette lighter receptacle in the motor vehicle to obtain electrical energy from the vehicle's electrical system to power these appliances. This energy may be used to directly power the appliances, thereby eliminating the battery pack otherwise used for power. Or the energy may be employed to charge the batteries in the pack.
Nearly all motor vehicles in use today have 12 volt electrical systems. However, different appliances are designed to be powered at different voltage levels, and seldom require exactly 12 volts. Moreover, different types of battery packs, say nickel/cadmium versus lead/acid, often require different charging voltages, even though rated for the same output voltage.
As a consequence of these different requirements, it is usually necessary to shift the 12 volt current obtained from the vehicle to another voltage before it can be utilized to power or recharge an appliance. Separate voltage shifting units have been devised for this purpose. It has also been proposed that the voltage shifting unit be incorporated into the power pack itself. Another approach has been to attach the voltage shifting unit to the electrical plug which is inserted into the cigarette lighter receptacle. A disadvantage of the latter arrangement is that other vehicle components in the vicinity of the cigarette lighter can interfere with insertion of the plug with the voltage shifting unit attached.